Aggregates and proppants are generally known in the art. An aggregate is a component of a composite material which provides certain properties to the composite material, including bulk and/or resistance to compressive stress. A proppant, which is a type of an aggregate, is a material used to hold open or “prop” an area in which the proppant is introduced. In the oil or gas industry, a proppant is typically used in association with hydraulic fracturing operations, and in sand control, such as in gravel-pack operations.
During the hydraulic fracturing process, a conductive fracture is induced underground in order to provide a path of extraction for a targeted subterranean material, such as a hydrocarbon, including oil or gas. Typically, a fracturing fluid is introduced into the targeted subterranean area. The fracturing fluid creates hydraulic fractures underground to the targeted subterranean materials. The hydraulic fractures provide a path for the targeted subterranean materials to be extracted, for example through an underground well. In order to keep the induced hydraulic fractures open, to maintain the fracture width, and/or to slow the decline of the fractures, a proppant is typically introduced into the hydraulic fractures. The proppant slows and/or inhibits closure of the fractures when the fracturing fluid is reduced. Accordingly, an appropriate proppant has the ability to flow into the fractures, the ability to form a “pack” or a partial monolayer that provides support and maintains the fractures in an open state, the ability to withstand substantial crushing force in the subterranean area (i.e. crush resistance), and the ability to facilitate flow of a hydrocarbon to an extraction bore or to a well head.
The volume and rate of hydrocarbon production through subterranean fractures or a wellbore can be a function of proppant conductivity. Proppant conductivity is the product of proppant permeability and fracture width. Hydrocarbon production rate can also be influenced by fracture length and the contact area of fractures with reservoir hydrocarbons. For example, an increase in proppant conductivity, fracture length, or fracture contact area with reservoir hydrocarbons can increase the hydrocarbon productivity of a well. Similarly, a decrease in proppant conductivity, fracture length, or fracture contact area with reservoir hydrocarbons can decrease the hydrocarbon productivity of a well.
Due to the necessary requirements of a proppant in hydraulic fracturing, only certain materials are suitable for use as a proppant. For example, some naturally occurring sands, known as “frac sand,” meet these requirements. Other materials used as a proppant include, but are not limited to, glass beads, steel shot, nut shells, ceramic pellets, synthetic resin pellets, sintered alumina or bauxite, a polymer, shells, or a mixture of any of these materials.
When designing or selecting a proppant, several proppant properties typically are taken into consideration, as the properties can affect proppant performance to achieve proppant conductivity, fracture length, and ultimately hydrocarbon production. As some of these properties can conflict with each other, the benefit and the cost typically needs to be considered prior to the design or selection of the proppant for a targeted application. In addition, the targeted application can vary depending upon certain factors of a well, including, but not limited to, formation type, formation depth, the treatment to be applied, and/or the equipment to be used.
For example, compressive forces in a fracture can often exceed 1,000 pounds per square inch or psi. A significant fraction of particulates making up a proppant can withstand these compressive forces without crushing or substantially breaking. A frac sand or a lightweight ceramic is often used in applications where compressive forces are less than about 10,000 psi, such as for relatively shallow wells. In deeper wells, where compressive forces can exceed 15,000 psi, higher strength proppants are typically used. These higher strength proppants are often composed of materials having a relatively higher specific gravity than other proppants, such as ceramic or bauxite.
The crushing of a proppant has certain disadvantages, including a reduction in fracture width or close and “pinch off” of a fracture, reducing proppant conductivity. In addition, fines generated from a crushed proppant can clog a proppant pack void space, reducing proppant pack permeability, and thus reducing proppant conductivity. Further, sharp-edged fines may be generated from a crushed proppant. These sharp-edged fines can concentrate the compression force onto an adjacent particle sphere, leading to the crushing of the adjacent particle and subsequent release of additional sharp-edged fines.
While a proppant having a higher specific gravity can improve crush resistance, transportability of the proppant is often compromised, requiring higher viscosity pumping fluids and/or higher pumping rates. In addition, proppants having a higher density generally have higher material costs. This is in addition to additional costs for larger pumping equipment and increased wear rates of fluid carrying equipment.
As another example, the size range of particles making up a proppant is typically relatively narrow and historically controlled through fractionation using sieves. The size range of particles is typically measured in terms of the diameter of the particles. An example of size range distributions of a proppant include, but are not limited to, 6/12, 8/16, 12/18, 12/20, 16/20, 16/30, 20/40, 30/50, 40/60, 40/70, 70/140, and 100 Mesh as according to U.S. sieve pan sizes used to fractionate the proppant. Narrower size range distributions of a proppant are commercially produced, for example for a ceramic proppant. For example, these narrower size range distributions may include 18/20, 20/30, and 30/40. Generally, a narrower size range distribution of a proppant maintained under stress can improve conductivity through increased proppant permeability. However, Median Particle Diameter (MPD) of a proppant can also significantly affect conductivity, as generally the larger the MPD, particularly when maintained under stress or pressure, the greater the conductivity.
A proppant having particles of a smaller MPD can exhibit a higher crush strength and a longer transport distance due to a reduced settling rate. Both of these factors can promote fracture productivity. However, an increase in fracture length and a corresponding increase in accessibility to reservoir hydrocarbons must be weighed against potentially reduced permeability and associated reduced conductivity of the proppant pack formed by these particles. A reduction in permeability and conductivity can reduce fracture productivity.
On the other hand, a proppant having particles of a larger MPD, particularly when maintained under stress, can exhibit relatively high permeability and high conductivity, promoting fracture productivity. However, these particles can settle relatively faster, compromising fracture length and potentially reducing accessibility to hydrocarbons and fracture productivity. Further, these particles can have a reduced crush resistance. Thus, upon crushing can reduce MPD, fracture width, reducing proppant permeability and conductivity.
As another example, the shape of particles in a proppant can profoundly impact its conductivity. Historically, proppants have been sought that have a spherical and rounded shape to maximize load bearing capacity and to even stress distribution, and maximize corresponding crush resistance, permeability, flowability, delivery distance within a fracture, effective fracture width through reduced embedment, and reduced pressure loss, tortuosity, friction against hydrocarbon flow, and abrasion. Together, these shape-dependent properties can serve to increase the effective conductivity of a proppant, and ultimately increase hydrocarbon production rates.
Packing together spheroidal or largely spherical and rounded particles can form capillary-like flow channels through a proppant matrix, leading to reduced tortuosity, and associated reduced pressure loss. This is of particular importance in areas of high flow rates, such as near a well bore or areas of fracture convergence. In these areas, fractures and fluid flow converge and Non-Darcy flow effects can be most pronounced. While spheroidal particles of uniform size offer excellent conductivity, these particles can be prone to flow-back into the well bore. Flow-back of a proppant is undesirable as it can reduce the volume of proppant in the fractures, reducing the productivity through the fractures. Further, proppant flowing back into the well bore and to the surface can abrade well bore components and surface equipment, leading to expensive equipment repair, equipments replacement, and costly downtime. Additional costs can be incurred for the removal and disposal of flow-back proppant from the oil and gas produced from the well bore.
As another example, the surface texture of particles in a proppant can impact proppant performance. A smooth surface texture can offer certain advantages, such as a reduced coefficient of friction. A reduced coefficient of friction can reduce flow friction, resulting in an increase in flow capacity of a fluid through a fracture. Conversely, irregularities on the outside surface of a proppant can lead to uneven stress distribution, proppant crushing, and fine generation. Further, surface irregularities can trap fracturing fluid used during injection, closing up a void space in a proppant pack and reducing proppant permeability and conductivity. This in turn reduces oil or natural gas production. Additionally, a prolonged clean-up of injection fluid can be expensive and cause delays in oil or natural gas production.
Surface irregularities, for example in the form of dents, protrusions, burs, rough surface textures, or angular edges has the additional disadvantage of a high degree of abrasiveness. The presence of an abrasive particle in the well bore during injection or production can damage well and pumping equipment, increasing tool and equipment costs and leading to costly well downtime. However, surface irregularities potentially can reduce proppant flow-back.
In addition, the presence of clusters in a proppant can have adverse affects on the proppant. A cluster is formed of many small granular particles, and has a rough surface texture. Clusters can reduce the strength of the proppant, increase flow friction, and ultimately reduce proppant conductivity. Clusters are often found in frac sand.
The presence of contaminant particles in a proppant can also have adverse affects on the proppant. Contaminant particles are often found in frac sand, and may include feldspar, mica, magnetite, hematite, biotite, milky quartz, iron ore, and/or dolomite. Contaminant particles can reduce proppant strength, increase acid solubility, increase abrasiveness, increase flow friction, and ultimately reduce proppant conductivity.
As another example, additional requirements for a proppant can include chemical inertness towards fracturing fluid crosslinkers and breakers, and acid tolerance.
Progress has been made to optimize functionality of certain synthetic proppants, such as ceramic proppants. For example, a lightweight ceramic proppant can have a relatively low specific gravity, and a high degree of sphericity and roundness. However, production costs of synthetic proppants can be high and further increased when the particle size distribution is narrowed. In addition, synthetic proppants can be highly abrasive and can incur additional costs related to equipment damage, tooling damage, and well downtime when used.
Frac sand, while relatively inexpensive, typically includes a heterogeneous mixture of particle shapes, which include irregularly shaped particles and highly spherical and rounded particles. Further, frac sand typically includes a heterogeneous mixture of particle surface textures, and may also include clusters and/or contaminants. Where some particles of frac sand have a smooth surface texture, other particles have a rough surface texture. Irregular or angular shaped frac sand particles, or particles having a rough surface texture can have a detrimental impact on conductivity, and ultimately can reduce the rate of hydrocarbon production. In addition, the abrasiveness of these frac sand particles incurs additional costs related to equipment damage, tooling damage, and well downtime when used.
Similar to frac sand, resin-coated frac sand or resin-coated sand includes a relatively heterogeneous mixture of particle shapes, including irregularly shaped particles and highly spherical and rounded particles. While a resin coating can slightly improve sphericity or roundness of a frac sand particle, significant irregularities in shape within the particle population remain. Resin-coated sand can be used near the well bore, a zone of high fluid velocity and turbulence, in order to reduce proppant flow-back into the well. A resin-coating can also reduce fine generations, and maintain a high structural integrity of proppant by improved crush resistance. This together acts to optimize conductivity and hydrocarbon flow through the well bore. However, resin chipping can lead to clogged void space, reduced permeability, and reduced strength of the resin-coated sand. In addition, resin-coated sand requires a costly special treatment which can be negatively affected by temperature.
Currently, no processing system exists that through direct modification can increase sphericity and roundness of frac sand to produce a generally highly spherical and rounded frac sand without also introducing surface irregularities or pre-stress particles of the frac sand. For example, while a sand reclamation system can be used to rub together frac sand particles in order to increase the sphericity and roundness of the particles, in doing so, dents and/or protractions can result on the surface of the particles. In another example of a system, a frac sand particle is repeatedly shot at high velocity against a metal plate to achieve a spherical and rounded particle shape. However, this process can lead to pre-stressing or fracturing of the particle, reducing crush resistance of the particle. In addition, in both system examples, significant waste is incurred during shape modification to the frac sand particles.
In addition, no processing system exists that can remove abrasive particles from a frac sand to produce an abrasion-resistant frac sand. An attrition scrubber can be used to remove a surface irregularity from a frac sand particle, reducing the roughness of surface texture and associated abrasiveness of the frac sand particle. However, an attrition scrubber is unable to significantly remove or affect relatively more angular, un-spherical, or irregularly shaped particles or clusters, or particles having a rough surface texture. These abrasive particles remain in the frac sand processed by an attrition scrubber.
Due to the disadvantages of irregularly shaped, rough surface texture sand, there is a need for a sand that is of highly spherical and rounded shape, is smooth of surface texture, yet retains the benefits of a low specific gravity. Further, a sand size gradation or MPD is currently not necessarily optimized for one or more proppant properties. There is a need for the ability to further modify a sand size gradation or MPD to result in optimal performance or economics of a proppant. In addition, there is a need for a more abrasion-resistant proppant that is less abrasive than a typical frac sand or a synthetic proppant, such as a ceramic proppant.
Furthermore, there is a need for a sand that exhibits greater permeability and conductivity, particularly for use near or adjacent to the well bore. This sand can be resin-coated, such as to further reduce flow-back, reduce fine generation, or to increase the strength of the sand. Furthermore, there is a need for a sand that would be an alternative to resin-coated sand, as the sand would not require resin-coating of particles, but that similarly reduces proppant flow-back.
In addition, due to the limited number of naturally-occurring aggregate particle deposits for certain uses, there is a need for a system of processing aggregate particles to acquire particles having certain desired properties. For example, there are a limited number of naturally-occurring aggregate particle deposits, such as sand, suitable for use as a proppant. As another example, there are a limited number of naturally-occurring aggregate particle deposits, such as sand, suitable for use in other industries, including, but not limited to, sand blasting, molding, shot peening, concrete, masonry, landscaping, agriculture, artificial turf, electronics, or filtration.
In addition, shipping of an aggregate particle can be expensive, and can economically limit access to certain aggregate particles. More specifically, while an aggregate particle of a distant deposit may have one or more beneficial properties, it can be cost prohibitive to ship compared to a local deposit. Accordingly, there is a need for a system of processing aggregate particles which may be movable.